people here dont understand the cap doesnt cause pressure, the heat does. heat is produced and maintained by thermostat opening and closing to route water to the radiator. rad cap is simply a safety device to allow pressure to escape if necessary. venting in a car ejects precious coolant and introduces air into the system; this is NOT the solution to lower pressure. lowering temperature is the ONLY way to reduce pressure of a ethylene glycol/water mixture. this total failure of people's understanding may be especially bad for the 540is(as they run hotter) and hit that 1.2 bars sooner then the i6 guys.

Finally, a ray of light. BMW owners seem (I do too at times) to jump at any perceived superior device than what BMW gave us. The pressure that the cap will hold will make absolutely no difference in how your car performs, how you will survive say a massive radiator leak while stuck in traffic, or in normal driving. You could put a 3 bar cap on with no difference in how the car performs, the temperature the car attains nor will it possibly save you in a failure. Where it may make a difference is if you accidentally over-fill the system to the point where the coolant has no place to expand to. The cap should (if the coolant reaches an excessive pressure) release the "extra" coolant. The tests performed here on this post demonstrate that this will exceed 1.2 bar or 1.4 bar and the longer you can keep your coolant in the closed system the better. Remember the cap is rated in pressure and no relation to temperature. A cooling system full of just water will increase pressure at a lower temperature than one filled with coolant. Cap Has no effect on temperature, only on pressure.